Apparatus for smoothing jagged border lines

ABSTRACT

Jagged border lines between image components of a reproduction image or the image of a display are smoothed visually at first by giving weight coefficients to a center pixel and the surrounding pixels thereof, and then by obtaining values for designating middle level densities to be used for the smoothing according to the sum of the coefficients.

FIELD OF THE INVENTION

This invention relates to an apparatus for smoothing jagged border linesbetween image components of a reproduction image recorded by using animage reproducing system such as a color scanner or a color displayingmonitor, particularly to such an apparatus for smoothing border linesbetween image components expressed in gradational density (brightness)values carrying a jagged or intermittent (when the border line is a thinenough) appearance.

BACKGROUND OF THE INVENTION

When an inclining line (for example, having the angle θ expressed by tanθ=0.25) must be displayed on a display capable of displaying an image ofgradational density (brightness) values, the line has conventionallybeen expressed as a black-and-white image as shown in FIG. 1, in whichthe line carries a jagged appearance in the sub-scanning direction (Y).

To smooth such a jagged border line, conventionally so-called a twinbeam method is put to practical use. In the method, a line expressed inblack-and-white is accompanied by a sub-line expressed in the middlelevel density (brightness) obtained from a computation when they arewritten into an internal memory of a display.

FIG. 2 shows an example of a black-and-white image having the sameinclination as the line shown in FIG. 1 displayed on a display by meansof the twin beam method.

Assuming that each of beam dots a, b, c, d . . . composing the incliningline (called a "main line" hereinafter) shown in FIG. 1 has a brightnesslevel of 5 (this number means the highest brightness), in the twin beammethod shown in FIG. 2, some of the brightness of each beam dotcomposing the main line is distributed to corresponding one ofaccompanying sub-beam dots a', b', c', d' . . . depending on theinclination and the position of each beam dot. In this, the brightnesslevel σ of a main beam dot and the brightness level σ' of itsaccompanying sub-beam dot amounts to the brightness level of 5=σ+σ' (forexample, if σ=4, then σ'=1) to smoothe the jagged appearance of theinclining line. The number deposited in each beam dot in FIG. 2indicates the brightness level thereof.

In the abovementioned twin beam method, the processing forimage-reproduction cannot be carried out in realtime, because thecomputation for determining proper brightness for all the main andsub-beam dots composing a black-and-white image is rather complicatedand time-consuming.

In another aspect, in a color scanner for reproducing images on aphotosensitive film, characters or drawing lines are usually recorded onthe scale of a drawing pixel which corresponds to one severalth of apictorial pixel (this is called a "high-resolution recording process").An apparatus for practicing such a high-resolution recording process canproduce a reproduction image containing rather smoothed border lines,however, the effect thereof is brought only to designated border linesand to black-and-white image components, not to border lines included inthe other pictorial components (such as continuous tone images).

SUMMARY OF THE INVENTION

It is an object of this invention to smooth all the jagged border linesbetween the image components of an output image by using a simpleapparatus.

It is another object of this invention also to carry out smoothingprocess onto the border line between two image components where thedifference of density (brightness) level between them are more than aspecific value.

To achieve the above objects, this invention comprises the followingfunctional steps.

At first, the density value of a pixel I_(m) (called a "center pixel"hereinafter) and the density values of its surrounding pixels aremultiplied by weight coefficients respectively to compute a mediatevalue S of all the (center) pixels. When the value S has a specificrelation to a fixed value R, a density value J computed from the value Sis output. When both values S and R are not in the specific relation,the density value V_(m) of the center pixel I_(m) is output.

In the above, a positive weight coefficient is given to the center pixelwhile a negative weight coefficient is given to each of the surroundingpixels so as to effect each other (to make the sum be "0"). Purportingto the surrounding pixels, for example the pixels composing asymmetrical cross in the main and the sub-scanning directionsintersecting at the center pixel can be adopted.

In embodiments of this invention, output density (brightness) valuesdepends on when 1S>R, 2-S>R or 3-S>R and S>R.

The above and other objects and features of this invention can beappreciated more fully from the following detailed description when readwith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows dots composing an inclining line carrying a jaggedappearance.

FIG. 2 shows dots composing an inclining line expressed by using a twinbeam method.

FIG. 3 shows the whole system of this invention.

FIG. 4 shows the detail of a jag suppressor of this invention.

FIG. 5 shows a part of another jag suppressor of this invention.

FIG. 6 shows a part of yet another jag suppressor of this invention.

FIG. 7 shows a jag suppression process of this invention.

FIG. 8 shows several types of electronic filters composed of weightcoefficients.

PREFERRED EMBODIMENT OF THE INVENTION

FIG. 3 shows a color scanner for printing to which the method of thisinvention is applied. An input scanning drum 2 is revolved by a motor 1,while an original picture mounted on the drum 2 is scanned by an inputscanning head 5 which is moved by a motor 4 along a feeding gear 6.Image signals obtained by scanning the original picture undergoanalog/digital conversion according to a sampling clock p which isobtained by processing multiple pulse output from a rotary encoder 3 inan input clock pulse producer 7. Thus digitized image signals of threecolor components R, G and B undergo processes of magnificationconversion, RGB/YMCK conversion, gradation correction, color correction,recording mode exchanging, sharpness emphasis and so forth in an imageprocessor 8 under the control of a control unit 18, and then input to ajag suppressor 9. The image signal undergo a jag suppression process ofthis invention in the jag suppressor 9 and then input to a recordingprocess unit 10 in which the image data are synchronized with arecording clock pulse output from a recording clock pulse producer 11,and used for generating corresponding halftone dot signals. Then thehalftone dot signals are used for driving a recording beam of arecording head 13 being moved along a feeding gear 14 by a motor 12 torecord a reproduction image onto a photosensitive film mounted on arecording drum 16 being revolved by a motor 15.

In FIG. 3, the input clock pulse producer 7 outputs a sampling clockpulse P and a single pulse Q, while the recording clock pulse producer11 outputs a recording clock pulse P₀ and a single pulse Q₀.

FIG. 4 shows the detail of the jag suppressor 9, in which the image dataare 8 bit data.

Density signal of a pixel of a scanning line is input to a latch 9₋₃₋₁synchronizing with the risetime of the clock pulse P, and then thesignal is input to corresponding address AS of a line buffer 9₋₁synchronizing with the falltime of the clock pulse P. Before that, theline buffer 9₋₁ already output the density signal of the pixel of theprevious scanning line stored in the address AS to a latch 9₋₃₋₃. Theimage data to the latch 9₋₃₋₃ are also input to the address AS of a linebuffer 9₋₂. Before that, the line buffer 9₋₂ already output the densitysignal of the pixel of the second previous scanning line stored in theaddress AS to a latch 9₋₃₋₆. Therefore, the image data of thecorresponding three pixels situated on three consecutive scanning linesare held on the latches 9₋₃₋₁, 9₋₃₋₃ and 9₋₃₋₆ at the same time. Andthen they are successively shifted to respective subsequent latchessynchronously (9₋₃₋₁ →9₋₃₋₂, 9₋₃₋₃ →9₋₃₋₄ →9₋₃₋₅ ; 9₋₃₋₆ →9.sub. -3-7).Consequently, at every falltime of the clock pulse P, the density valueV_(m) of a center pixel I_(m) is output from the latch 9₋₃₋₄, and thedensity values V_(b) and V_(c) of the surrounding pixels I_(b) and I_(c)(adjoining to the center pixel I_(m) in the sub-scanning direction) areoutput from the latches 9₋₃₋₂ and 9₋₃₋₇, while the density values V_(a)and V_(d) of the surrounding pixels I_(a) and I_(d) (adjoining to thecenter pixel I_(m) in the main scanning direction) are output from thelatches 9₋₃₋₅ and 9₋₃₋₃.

Thus obtained density values V_(a), V_(b), V_(c) and V_(d) of thesurrounding pixels I_(a), I_(b), I_(c) and I_(d) are input to anaveraging circuit 9₋₄ to be averaged, which outputs the average value(V_(a) +V_(b) +V_(c) +V_(d))/4 to the negative terminal of a subtractor9₋₅. On the other hand, the density value V_(m) of the center pixelI_(m) is input to the positive input-gate of the subtractor 9₋₅. Thesubtractor carries out a computation:

    S=V.sub.m -1/4(V.sub.a +V.sub.b +V.sub.c +V.sub.d)         (1)

wherein the value S is a mediate value for obtaining a middle leveldensity value J (mentioned later). In this, the second term of the rightmember means that the sum of the density value of each surrounding pixelis multiplied by a weight coefficient of -1/4 as shown in FIG. 8(a).

Thus obtained mediate value S is inverted in an inverter 9₋₆ to be aninverted value -S, and output to a multiplier 9₋₇. Meanwhile, aspecified value U is registered in a register 9₋₁₃ beforehand and isinput to the multiplier 9₋₇. In the multiplier 9₋₇, the inverted value-S is multiplied by the value U and the resultant value is output to anadder 9₋₁₀. The adder 9₋₁₀ adds the resultant value to the value V_(m)being input from the latch 9₋₃₋₄ to produce the value J:

    J=V.sub.m +(-S)·U                                 (2)

wherein the value J is a middle level density value to be input to aselector 9₋₁₁.

Said inverted value -S is also input to a comparator 9₋₉, to which afixed value R registered in a register 9₋₈ is input beforehand. Thecomparator 9₋₉ compares the value -S to the value R. When -S>R, thecomparator 9₋₉ outputs a selection signal T of "H" (high level) to theselector 9₋₁₁ to make it output the middle level density value J. When-S≦R, the comparator 9₋₉ outputs a selection signal T of "L" (low level)to the selector 9₋₁₁ to make it output the value V_(m) of the centerpixel I_(m). In this manner, the output value of the selector 9₋₁₁ isinput via a latch 9₋₁₂ to the recording process unit 10.

FIG. 7(a) shows a jagged border line on a conventional black-and-whiteimage (the thick portion is expressed by "black-level (1)", while thethin portion is expressed by "white-level (0)").

Under a condition in which the value U to be set up to the register 9₋₁₃is 4/3, and the value R to be set up to the register 9₋₈ is 0, the imageof FIG. 7(a) is processed as follows.

The mediate values S (S.sub.α, S.sub.β, S₆₅ and S.sub.δ) for pixels α,β, γ and δ of FIG. 7(a) are:

    S.sub.α =1-1/4·(1+1+0+1)=1/4·1

    S.sub.β =1-1/4·(0+1+0+1)=1/2·1

    S=0-1/4·(0+1+0+1)=-1/2·1

    S=0-1/4·(0+1+0+0)=-1/4·1

according to the equation (1), wherein 1 represents the density ofblack-level and 0 represents the density of white-level, and in thisexample, 0 is assumed zero.

When the value S is zero or positive (S≧0=R), the comparator 9₋₉ outputs"L" signal to the selector 9₋₁₁ to make it output the main pixel valueV_(m) =1. When the value S is negative (S<0=R), the comparator 9₋₉outputs "H" signal to the selector 9₋₁₁ to make it output thecorresponding middle level density value J.

In this, when the value S=-1/4·1, the value J isJ=0-(-1/4·1×4/3)=-1/3·1; when S=-1/2·1, the value J isJ=1-(-1/2·1×4/3)=2/3·1 according to the equation (2).

When the black-level (1) corresponds to the density of 100%, the pixelof value S=-1/4·1 is recorded in the density of about 33% (J=1/3·1),while the pixel of value S=-1/2·1 is recorded in the density of about67% (J=2/3·1). Therefore, an image shown in FIG. 7(c) can be obtained,wherein the jagged appearance on border lines are eased visually.

FIG. 5 shows another embodiment of this invention, from which the dataorder regulating means and the averaging circuit 9₋₄ are eliminated forexplanation because they are as same as the embodiment shown in FIG. 4.In the embodiment of FIG. 5, the mediate value S is directly input tothe comparator 9₋₉ to be compared with the value R. When S>R, theselector 9₋₁₁ outputs the middle level density value J. When S≦R, theselector 9₋₁₁ outputs the density value V_(m) of the center pixel I_(m).

The black-and-white image of FIG. 7(a) is processed by the circuit ofFIG. 5 as in the following way.

The value S has the same value as shown in FIG. 7(b). Assuming that thevalue R is zero and when S≦0=R, the center pixel value V_(m) (1 or 0) isdirectly output. When S>0=R, the middle level density value J is output.

When the black-level (1) corresponds to the density of 100%, the pixelof value S=1/2·1 is recorded in the density of about 33% (J=1/3·1),while the pixel of value S=1/4·1 is recorded in the density of 67%(J=2/3·1). Consequently, the image shown in FIG. 7(d) can be obtained.

FIG. 6 shows yet another embodiment of this invention, of which circuitstructure is a combination of the circuits of FIGS. 4 and 5. That is, acomparator 9₋₉₋₁ compares the inverted value -S obtained from theinverter 9₋₆ with a fixed value R₁ stored in a register 9₋₈₋₁ andoutputs the selection signal T₁ of "H" when -S>R₁, or outputs the signalT₁ of "L" when -S<R₁. In the meantime, a comparator 9₋₉₋₂ compares thevalue S obtained from the subtractor 9₋₅ with a fixed value R₂ stored ina register 9₋₈₋₂ and outputs the selection signal T₂ of "H" when S>R₂,or the signal T₂ of "L" when S<R₂.

Consequently, the selector 9₋₁₁ outputs the density value V_(m) of thecenter pixel I_(m) according to the selection signal of "L" when -R₁≦S≦R₂. Or the selector 9₋₁₁ outputs the middle level density value Jaccording to the selection signal of "H" when R₁ <-S or S>R₂.

When the image of FIG. 7(a) is processed by the circuit of FIG. 6, thevalue S of each pixel is as shown in FIG. 7(b). Assuming that the valuesU, R₁ and R₂ are U=2/3 and R₁ =R₂ =1/4, a selection signal T₃ from aOR-gate 9₋₁₄ becomes "L" when the pixel of which value S is S=0 orS=±1/4·1 is processed to make the selector output the density valueV_(m) of the center pixel I_(m). When the pixel of which value S isS=±1/2·1 is processed, the signal T₃ becomes "H" to make the selectoroutput the middle level density value J. The value J becomes J=1/3·1 forthe pixel of which value S is S=-1/2·1, or becomes J=2/3·1 for the pixelof which value S is S=+1/2· 1 as shown in FIG. 7(e).

In the circuit of FIG. 6, when the image of FIG. 7(a) is processed undera condition in which the value U, R₁ and R₂ are U=4/3, R₁ =0 and R₂ =1,the image of FIG. 7(c) is recorded. When the image of FIG. 7(a) isprocessed under a condition in which the value U, R₁ and R₂ are U=4/3,R₁ =1 and R₂ =0, the image of FIG. 7(d) is recorded. Therefore, byvarying the values U, R₁ and R₂, the position of a border line can beshifted. When the portion of black-level "1" is a ruled line, thethickness thereof can be varied.

When the image of FIG. 7(a) is processed by the circuit of FIG. 6 undera condition wherein U=4/3, R₁ =0, and R₂ =0, the middle density value Jbecomes J=5/6·1, J=2/3·1, J=1/3·1 and J=1/6·1 when the pixels of whichvalues S are S=1/4·1, S=1/2·1, S=-1/2·1 and S=-1/4·1 respectively. Thatis, when the value J takes more minute value as mentioned above, theborder line recorded under the condition is smoothed more than theborder lines of the images shown in FIGS. 7(c)(d) and (e). Therefore, byvarying the values U, R₁ and R₂, a desired density gradient can beobtained.

The abovementioned circuits adopt a cross-shape electronic filter inwhich the center pixel is given the weight coefficient of 1 while eachof four surrounding pixels are given that of -1/4 as shown in FIG. 8(a).

As is obvious from the above example, the weight coefficient of thecenter pixel and the sum of the coefficients of the surrounding pixelsoffset each other, which fact can pass for any electronic weightcoefficient filters of this invention.

FIG. 8(b) shows another cross-shape imagenary filter in which the centerpixel is given the weight coefficient of 1, while each of foursurrounding pixels situated in the main scinning direction and foursurrounding pixels situated in the sub-scanning direction symmetricallyabout the center pixel is given a weight coefficient of -1/8. FIG. 8(c)shows another cross-shape electronic filter in which the center pixel isgiven the weight coefficient of 1, while each of six surrounding pixelssituated in the main scanning direction and six surrounding pixelssituated in the sub-scanning direction symmetrically about the cinterpixel is given a weight coefficient of -1/12.

FIG. 8(d) shows another cross-shape imaginary filter in which the centerpixel is given the weight coefficient of 1, while each of twosurrounding pixels situated in the main scanning direction and foursurrounding pixels situated in the sub-scanning direction symmetricallyabout the center pixel is given a weight coefficient of -1/6.

Generally speaking, when twenty five pixels α₁ to α₂₅ arranged in matrixas shown in FIG. 8(e) are to be given weight coeficients, the weightcoefficient for the center pixel α₁₃ and the sum of the weightcoefficient for surrounding pixels α₁ . . . α₁₂, α₁₄ . . . α₂₅ mustoffset each other (amount to zero).

When the difference of density between the portion of black-level "1"and that of the portion of white-level "0" as shown in FIG. 7(a) islittle, no preventive measures need not be taken for the appearance ofjagged border lines. In such a case, by using the circuit as shown inFIG. 6 under a condition in which the values U, R₁ and R₂ are U=2/3, R₁=1/2 and R₂ =1/2, the selector 9₋₁₁ can output a signal of the sameimage as FIG. 7(a).

When said difference of density is little enough, it would rather benecessary to carry out a sharpness emphasis on the border line by theimage processor 8 than to carry out the jag suppression process thereto.

As mentioned above, a portion of less contrast of density had betterundergo the sharpness emphasis process, while a portion of more contrastof density had better undergo the jag suppression process to improve thequality of an image to be reproduced.

Although in the abovementioned embodiment, the sharpness emphasisprocess is carried out previously in the image processor 8 before thejag suppression process, the order can also be reversed.

The method of this invention can be applied to a displaying monitor aswell as to the abovementioned image reproducing system.

The method of this invention is capable of realizing a jag suppressionprocess and smoothing process by means of a simple apparatus andalgorithm, which have conventionally been very complicated. Therefore,when the method of this invention is applied to a displaying monitor, aneconomical displaying monitor of high resolution power can be realized.When it is applied to an image reproducing system, an economical imagereproducing system capable of suppressing the appearance of jagged orintermittent lines can be obtained.

I claim:
 1. A method for smoothing a jagged border line between imagecomponents of a reproduction image, comprising the steps of:(a)obtaining a sum S of a density value of a center pixel to which apositive weight coefficient is given and density values of itssurrounding pixels to which negative weight coefficients are given; (b)obtaining a value J according to the sum S; (c) comparing the sum S witha predetermined fixed value R; (d) selecting one of a density valueV_(m) of a center pixel I_(m) and the value J obtained in the step (b)depending on a result of step (c) and providing the related densityvalue to said center pixel; and (e) repeating steps (a)-(d) for anothercenter pixel to smooth the border line.
 2. A method claimed in claim 1in which the surrounding pixels comprise an arbitrary number of pixelswhich form a cross having components lying respectively in main andsub-scanning directions and intersecting at the center pixel.
 3. Amethod claimed in claim 1 in which the number of the surrounding pixelsin the main scanning direction is the same as that of the surroundingpixels in the sub-scanning direction.
 4. A method claimed in claim 3 inwhich the number of the surrounding pixels in the main scanningdirection is 2 and the number of the surrounding pixels in thesub-scanning direction is
 2. 5. A method claimed in claim 1 in which apositive weight coefficient given to the center pixel and the sum of thenegative weight coefficients given to a surrounding pixels offset eachother.
 6. A method claimed in claim 1 in which the weight coefficientfor the center pixel is 1 and the weight coefficient for each of foursurrounding pixels composed of two pixels in a main scanning directionand two pixels in a sub-scanning direction symmetrical about the centerpixel I_(m), is -1/4.
 7. A method claimed in claim 1 in which the valueJ is derived from subtracting the product of the value S and a specifiedvalue U from the density value V_(m) of the center pixel I_(m).
 8. Amethod claimed in claim 1 in which the value J is output when the valueS is greater than the fixed value R and the value V_(m) is output whenthe value S is equal to or smaller than the value R.
 9. A method claimedin claim 1 in which the value J is output when the inverted value -S isgreater than the fixed value R and the value V_(m) is output when theinverted value -S is equal to or smaller than the value R.
 10. A methodclaimed in claim 1 in which the value J is output when the invertedvalue -S of the value S is greater than the value R₁ or when the value Sis greater than the value R₂, and the value V_(m) is output when theinverted value -S of the value S is equal to or smaller than the valueR₁ or the value S is equal to or smaller than the value R₂.
 11. A systemfor smoothing a jagged border line between image components of areproduction image, comprising:(i) means for scanning pixels of thereproduction image and for defining center pixels of a border betweenimage components; (ii) first means for obtaining a sum S of densityvalues given to each center pixel and to surrounding pixels thereof;(iii) second means for obtaining a value J according to the sum S; (iv)comparator means for comparing the sum S with a fixed value R; and (v) athird means responsive to said comparator means for selecting one of adensity value V_(m) of the center pixel I_(m) and the value J to beprovided to said center pixel to smooth the border line.
 12. A systemclaimed in claim 11 in which the first means comprises:(i) an averagingcircuit which computes the average of the weight coefficient given toeach of the surrounding pixels, said surrounding pixels comprisingpixels of the same number in both main and the sub-scanning directionssymmetrical about the center pixel; and (ii) a subtracting circuit whichsubtracts the averaged value from the density value V_(m) of the centerpixel I_(m).
 13. A system claimed in claim 12 in which the averagingcircuit is a circuit for obtaining an average of the weight coefficientsgiven to a of two by two array of said surrounding pixels.
 14. A systemclaimed in claim 11 in which the second means comprises:(i) amultiplying circuit for multiplying a value -S obtained by inverting thesum S and a specified value U to obtain a value -S.U; and (ii) an addingcircuit for adding the obtained value -S.U to the value V_(m) of thecenter pixel I_(m).
 15. A system claimed in claim 11 in which the thirdmeans includes means for outputting one of an value J when the invertedvalue -S is greater than the value R, and the value V_(m) of the centerpixel I_(m) when the inverted value -S is equal to or smaller than thevalue R.
 16. A system claimed in claim 11 wherein the third meansincludes means to output one of (i) the value J when the value S isgreater than the value R, and (ii) the value V_(m) of the center pixelI_(m) when the value S is equal to or smaller than the value R.
 17. Asystem claimed in claim 16 wherein the third means includes means tooutput the value J when an inverted value -S is greater than a fixedvalue R₁ or when the value S is greater than a fixed value R₂, or (ii)the value V_(m) of the center pixel I_(m) when the inverted value -S isequal to or smaller than the value R₁ or when the value S is equal to orsmaller than the value R₂.